Entrance animation: *pops into existence with a surprised/spooked look on his face* “Steve joined the battle!”
Neutral B - Build - Steve could select + place certain blocks, which would be limited in amount he can have and place, the damage they can take, and the time they stay on stage. When he runs out, he can get more by attacking opponents.
Up B - Ender Pearl - Steve can throw an ender pearl upwards/at an angle. Wherever the ender pearl lands, he’ll teleport there. He can also teleport to the ledge of the stage if he can, and directly to opponents if the ender pearl hits them.
Side B - Minecart or Elytra Rocket - two ideas. Minecart would send Steve flying forwards in...uh...the minecart. He can’t jump with the cart, and turning around will kick him out and send the cart flying a bit forwards. He can ride offstage, and jump out whenever he wants; a powered rail is spawned under the cart wherever Steve uses it, as an aesthetic feature that’s just the reason why the cart gets launched.
Elytra Rocket would be similar to Charizard’s side-B; Steve puts on a pair of elytra (wings), and readies a firework, launching himself sideways and he’ll spin while in motion, damaging opponents and launching them upwards with the spin. If the thumbstick is held sideways, he’ll stop spinning after a moment and glide downward, but he can’t fly back up; he can slow his descent if you move the thumbstick upwards, however, by facing up and kicking his legs, his elytra still spread out, but doing this hurts his horizontal speed. He’ll automatically stop using it after a couple of seconds, but you can jump out of it before then; at the end, though, he’ll be put into freefall (maybe? I don’t know, maybe it’d be fine without freefall).
Down B - Craft - Steve takes out a crafting table, and can use resources he can get from attacking opponents to craft better gear. He can give himself armor, or better tools for some of his moves. The upgrades would be temporary, disappearing after some uses / hits taken. He can also bring out a brewing stand or enchanting table, letting him make potions/splash potions or further boost his gear. Basically, he starts out a bit weaker than other fighters, and this helps him grow stronger. When he brings out one of these blocks, he’ll throw it on the ground to deal damage to anyone around him, and can deal damage while crafting by quickly moving his arms around.
Final Smash - Withering End - Steve summons an End Portal in front of him, and opponents hit will teleport to the End. He will then quickly summon the Wither next to the opponents, which will create an explosion after a short moment and blast the opponents with Wither skulls. The Ender Dragon will then fly down after shooting a “fire”ball (it’s actually purple and creates a purple AoE cloud that deals damage), breathe its purple, damaging breath on the opponents and finally fly into them, sending them flying back out of the portal.
Steve’s normal attacks could use many different items/blocks from Minecraft. His sidesmash could be a heavy pickaxe swing downwards, which could spike opponents; his side-tilt could be a sweep with a hoe or maybe opening/shutting a chest (could absorb projectiles while open). His down-air could be dropping primed TNT or an anvil...and there’s plenty of other stuff he could use, such as a bow, a crossbow, a trident, potions, splash potions, lingering potions, swords, axes, shovels, pistons (I think pistons could be used in throws), spawn eggs (could summon different mobs), slime blocks, honey blocks, and other stuff.